1. Field of the Invention
This invention relates to improvements in a toughened glass sheet which has a thickness of, for example, from 2.5 to 3.1 mm and is tempered by air-cooling in order to provide a toughened glass sheet high in tempered degree and suitable for architectural windowpanes and automotive vehicle windowpanes.
2. Description of the Prior Art
Hitherto a variety of methods of toughening or tempering a glass sheet having a thickness ranging, for example, from 2.5 mm to 3.5 mm by air-cooling have been proposed and put into practical use in order to obtain a toughened glass sheet. One of them is disclosed in Japanese Patent Provisional Publication No. 52-121620 which describes a glass sheet and a method of producing it. The glass sheet is used as a side windowpane and a rear windowpane of an automotive vehicle and locally chilled so that regions low in tempered degree are distributed among regions high in tempered degree. The average central tensile stress of the glass sheet is set within a range of from the maximum value of 62 MN/m.sup.2 in all glass sheets having a thickness of 2.5 to 3.5 mm to the minimum value of 56.5 MN/m.sup.2 in a glass sheet having a thickness of 2.5 mm or the minimum value of 53 MN/m.sup.2 in the glass sheet having a thickness of 3.5 mm upon being changed inversely with the thickness. Regions uniform in principal stress to be applied in flat plane of the glass sheet are distributed throughout the glass sheet, in which the maximum value of principal stress difference in at least some of the above regions are fallen within a range of from 8 to 25 MN/m.sup.2. Accordingly, almost all principal stresses in regions adjacent the regions exhibiting the maximum value of the principal stress difference are directed in a variety of directions, in which the distance between the centers of the adjacent regions is within a range of from 15 to 30 mm.
Another one is disclosed in Japanese Patent Provisional Publication No. 54-33517 which describes a method of toughening a glass sheet and a toughened glass sheet produced thereby. In this method, tempering gas stream is a single local gas stream which is pulsed at a repeated frequency in connection with a velocity at which the glass sheet is carried to a tempering station thereby forming highly tempered regions distributed in a lowly tempered regions. Additionally, gas jets are directed to cross the moving direction of the glass sheet and arranged separate from each other in which gas stream of each gas jet is pulsed, thus applying many local gas stream to the glass sheet.
A further one is disclosed in Japanese Patent Provisional Publication No. 60-145921 which describes a method of toughening a glass sheet and an apparatus therefor. In this method, gas is blown to the opposite surfaces of a glass sheet thereby tempering the glass sheet, in which the maximum pressure drop of gas occurs at the free end of the nozzle. Additionally, this Publication describes the following techniques: The gas is supplied to the nozzle at a flow velocity which becomes at least sonic velocity at the outlet of the nozzle. The end of the nozzle is disposed close to the glass sheet and located within such a distance from the glass sheet as to be 6 times the diameter (width) of the gas jet stream at the outlet of the nozzle. The nozzles are arranged in such a pattern that the distance among the nozzles is shorter at the starting part of the tempering station in the moving or longitudinal direction of the glass sheet than that at the terminal part of the tempering station. The amount of gas to be blown to the glass sheet is larger at the starting part of the tempering station than that of the other parts of the tempering station. Throttle nozzles are used to enable air to be ejected at sonic velocity.
A further one is disclosed in Japanese Patent Provisional Publication No. 60-103043 which describes a method of toughening a glass sheet and an apparatus therefor. In this method, ejection steam of fluid is blown from nozzles to at least one surface of the glass sheet thereby tempering the glass sheet, in which gas is supplied to the nozzle at a velocity which allows the gas to flow at at least sonic velocity at the outlet of the nozzle. The above ejection stream is a mixture of the gas and sprayed liquid. Additionally, this Publication describes that the liquid is atomized by being supplied to impulse wave generated inside the nozzle.
Further ones have also proposed by the same group as the inventors of the present invention of this application. One of them is disclosed in Japanese Patent Publication No. 62-50416 (U.S. Pat. No. 4,662,926) which describes a method of toughening a glass sheet. In this method, cooling nozzles of a blast head are arranged as follows: A single center of concentrical circles is set at a nozzle installation face of the blast head, and the cooling nozzles are located along the concentrical circles and aligned radially in which the distance among the nozzles are not larger than 50 mm. Cooling medium is blown from the thus arranged cooling nozzles onto the glass sheet thereby tempering the glass sheet.
The same group have further proposed similar methods similar to that of Japanese Patent Publication No. 62-50416, as disclosed, for example, in Japanese Patent Publication No. 62-38289, Japanese Patent Publication No. 62-47819, Japanese Patent Publication No. 62-51212, and Japanese Patent Publication No. 63-43327. Now, Japanese Patent Publication No. 62-38289 describes that at least one of a pair of pressing dies is provided with heating elements which are arranged to concentrically spread throughout a range of from the center to the outside. Japanese Patent Publication No. 62-47819 describes that a plurality of baffle plates are arranged to concentrically spread throughout a range of from the center toward the outside of the cooling nozzle installation surface of the blast head. Japanese Patent Publication No. 62-51212 describes that at least one of a pair of pressing dies is provided with cooling elements which are arranged to concentrically spread throughout a range of from the center toward the outside so as to conduct a preliminary cooling, and then quenching the glass sheet by blowing a cooling medium to the glass sheet. Japanese Patent Publication No. 63-43327 describes that cooling nozzles are arranged along concentric circles and to concentrically spread throughout a range of from the center toward the outside of the cooling nozzle installation surface of the blast head, in which the cooling nozzles other than the center cooling nozzle are arranged such that their axes are inclined 3 to 45.degree. in a tangential direction of the concentric circles relative to a vertical axis which is perpendicular to the blast head surface in order to quench the glass sheet.
The same group has proposed a further glass sheet toughening method as disclosed in Japanese Patent Publication No. 4-29615 (U.S. Pat. No. 4,735,646) which describes the following technique: In tempering a glass sheet having a thickness of, for example, 1.5 to 3.0 mm, the pressure of cooling air is rapidly lowered from a gauze pressure of 2 to 8 kg/cm.sup.2 to a gauze pressure of 0.05 to 0.5 kg/cm.sup.2 to be fed into an air chamber, upon which the glass sheet is quenched by using like a shock tube a range of from the inside of the air chamber to the tip end of the cooling nozzle. The cooling air is generated by an air compressor.
Furthermore, the same group has proposed similar techniques as disclosed in Japanese Patent Publication No. 6-2593, Japanese Patent Publication No. 6-23068 (British Patent No. 2,232,978), Japanese Patent Publication No. 624995 (U.S. Pat. No. 5,112,377), and Japanese Patent Provisional Publication No. 3-228841 (U.S. Pat. No. 5,127,932). Now, Japanese Patent Publication No. 6-2593 describes a heat treatment method for glass articles, in which the pressure of cooling air is rapidly lowered from a gauze pressure of 1.5 to 8 kg/cm.sup.2 to a gauze pressure of 0.01 to 0.2 kg/cm.sup.2 to be fed into an air chamber. In this process, the glass article is quenched by using like a shock tube a range of from the inside of the air chamber to the tip end of the cooling nozzle, and additionally cooling is gradually made from the central part to the peripheral part of the glass article with a time lag. Japanese Patent Publication No. 6-23068 describes a two-stage cooling technique in which air (generating shock wave) having a heat transfer coefficient of 300 to 1000 kcal/m.sup.2 .multidot.h.multidot..degree.C. is blown to a glass article, and thereafter air having a heat transfer coefficient of 100 to 300 kcal/m.sup.2 .multidot.h.multidot..degree.C. is blown to the glass article. Japanese Patent Publication No. 6-24995 describes that quenching a glass sheet is accomplished with nozzles N.sub.C connected to an air compressor and nozzles N.sub.B connected to a blower, in which cooling air from the nozzles N.sub.C to be blown to the glass sheet to be tempered contains air exhibiting shock wave, and the pressure of the nozzles N.sub.B and the pressure of the nozzles N.sub.C are in a relation of 0&lt;(P.sub.B -P.sub.C).ltoreq.500 mmAq. Japanese Patent Provisional Publication No. 3-228841 describes that after a preliminary cooling is made by blowing air in a stripes pattern whose stripes has an inclination of .theta. (45.degree.&lt;.theta..ltoreq.90.degree.) relative to the moving direction of the glass sheet, cooling medium is blown to quench the glass sheet.
However, difficulties have been encountered in the above conventional techniques, as set forth below. The glass sheets according to Japanese Patent Provisional Publication No. 52-121620 and Japanese Patent Provisional Publication No. 54-33517 are insufficient in strength as a windshield or windowpane of an automotive vehicle. They are assumed to meet some requirements of automotive glass test standards, such as the number of fragments and no existence of sharp edge having a length not less than 100 mm in a fragmentation test. However, they are difficult to securely meet the requirements of the height of not less than 2 m in an impact resistance test. The above Publication does not describe such an impact resistance.
According to Japanese Patent Provisional Publication Nos. 60-145921 and 60-103043, the cooling rate of the glass sheets is increased to enhance the tempering by producing the sonic velocity air ejection or the impulse wave. These glass sheets are difficult to securely meet the requirements of automotive glass test standard, for example, of the height of not less than 2 m in the impact resistance test. Such an impact resistance is not described in these Publications.
Additionally, according to the glass sheet toughening methods proposed by the same group as the present inventors and disclosed in Japanese Patent Publication Nos. 62-38289 and 4-29615, the glass sheets are tempered by providing difference in cooling to the glass sheet in a concentric manner or by cooling the glass sheet in a shock tube manner. By these tempering, the glass sheets can be largely improved so as to meet the requirements of the automotive glass test standard, for example, the number of fragments, no production of sharp edge having a length exceeding 100 mm, and no production of fragment having an area of not less than 3 cm.sup.2 in the fragmentation test. However, concerning the impact resistance test, the glass sheets according to the Publications are improved so as to meet the requirements of automotive glass test standard of the height ranging from 1.5 to 2 m but cannot meet the requirement of the height of not less than 2 m.